That is what probably caused the dungeon to be reset. You went to the Library and bought a treasure card.

It sounds like you broke the line in your questing. There are limits to what you can do when you leave a dungeon.

If you still think that you should get credit for the dungeon (if you haven't already gone through it again), you might want to send an e-mail to help@kingsisle.com and explain this to them. Be sure to give them your UserName in the game, and the name of the wizard who had this problem.